The present invention relates to methods and apparatus for maintaining synchronization between a control circuit and a rotor of a polyphase motor during power interruptions, particularly when rotor position sensors are not employed in the control and drive of the polyphase motor.
Polyphase AC motors, such as permanent magnet, synchronous machines must be driven such that the windings thereof are energized as a function of the rotor position (and, thus, the rotor flux) in order to obtain driving torque from the machine. Conventionally, the rotor position is obtained by way of one or more rotor position sensors within the polyphase motor assembly, which sensors provide signals indicative of the rotor position to a control circuit.
The material and labor costs associated with employing position sensors within the polyphase motor assembly are undesirable and, therefore, techniques have been developed that permit proper energization of - the windings of a polyphase motor without using position sensors. Some of these techniques are discussed in, for example, U.S. Pat. Nos. 5,565,752; and 5,929,577, the entire disclosures of which are hereby incorporated by reference.
Control and drive techniques that do not require position sensors share a common characteristic, namely, that the rotor position of the polyphase motor is unknown at startup. In order to deal with the unknown rotor position, these techniques employ an open-loop acceleration process where the windings of the polyphase motor are driven without synchronization with the rotor position until the motor reaches a threshold rotational speed. At this speed, the polyphase motor generates signals of sufficient magnitudes to provide an indication of the rotor position. Among the signals that may be indicative of the rotor position are the back electromotive force (BEMF) voltages of the windings, the winding currents, etc.
Reference is now made to FIG. 1, which illustrates a block diagram of a conventional system 10 for controlling and driving a polyphase motor 18, which system measures the BEMF voltages of the polyphase motor 18 to determine rotor position. The system 10 includes a DC source 12, a control circuit 14, a driver circuit 16, and the polyphase motor 18. The DC source 12 produces a voltage, VDC, with respect to ground, which is utilized to provide an operating DC voltage, VCC, to the control circuit 14 and to provide a DC bus voltage, VBUS, to the driver circuit 16. The control circuit 14 provides commutation control signals to the driver circuit 16 such that the driver circuit 16 can properly energize the windings of the motor 18. The windings of the motor 18 (which are typically in the standard wye configuration, but which may also be in the delta configuration) are coupled to the driver circuit 16 by way of nodes A, B, and C. The driver circuit 16 provides various current paths among these nodes, the DC bus, and ground in order to drive the polyphase motor 18. The control circuit 14 monitors the voltages at nodes A, B, and C, such as the BEMF voltages, and utilizes same to maintain synchronization with the rotor position of the polyphase motor 18.
Unfortunately, the conventional techniques of monitoring signals indicative of rotor position (such as the BEMF voltages) cannot maintain synchronization with the polyphase motor 18 in the event of a power interruption, even if the power interruption is only momentary and the motor 18 has not stopped turning. This is so because during the power interruption the control circuit 14 is de-energized and looses all synchronization information. This is best seen in FIG. 2, which is a graphical representation of the characteristics of the voltage at node A, the DC bus voltage, and the DC source voltage during a power interruption. At time t0, a power interruption occurs and the DC source voltage, VDC, falls from about 24 volts to about 0 volts. Assuming that there is some impedance between the DC source 12 and the DC bus, the DC bus voltage, VBUS, (and VCC) falls after t0 as a function of the speed of the polyphase motor 18, which is decelerating. Likewise, the voltage at node A falls as a function of the slowing rotational speed of the polyphase motor 18. When the operating DC voltage, VCC, has fallen below, for example, about 15 volts, the control circuit 14 ceases to function properly and loses synchronization with the rotor position of the polyphase motor 18.
When power is restored, resynchronization of the control circuit 14 to the rotor position must be established in order to properly commutate the windings of the polyphase motor 18. Among the conventional processes for reestablishing synchronization is permitting the polyphase motor 18 to stop rotating and restarting the polyphase motor 18 utilizing the open-loop acceleration process discussed above. This technique may be unsatisfactory for various reasons, including the delays associated with stopping and restarting the polyphase motor 18, which are exacerbated when the inertias of the motor load and/or the rotor itself are large.
Other techniques have been developed for reestablishing synchronization between the control circuit and the rotor position, which techniques are set out in detail in U.S. Pat. Nos. 5,223,772; 5,172,036; and 6,194,861, the entire disclosures of which are hereby incorporated by reference. These conventional techniques, however, all presuppose that synchronization has been lost and must be reestablished using some specialized process. The manifest disadvantage of these techniques, therefore, is the reactive approach that they take to the loss of synchronization. Indeed, they do not address the root problem: the loss of synchronization itself.
Accordingly, there are needs in the art of new methods and apparatus for maintaining synchronization between a control circuit and a rotor of a polyphase motor during power interruptions, so long as the motor is rotating.
In accordance with one or more aspects of the present invention, a method includes: monitoring a level of a DC source that is used to provide an operating DC voltage to a control circuit and to provide DC bus voltage to a driver circuit for a polyphase motor, the control circuit being of a type that senses signals in windings of the polyphase motor to determine a rotor position thereof and to maintain synchronization therewith; converting kinetic energy of the polyphase motor into a secondary DC source when the level of the DC source has fallen and reached a threshold; and regulating a voltage level of the secondary DC source such that it is operable to provide the operating DC voltage to the control circuit, and such that the control circuit is capable of maintaining synchronization with the polyphase motor while the DC source remains substantially at or below the threshold.
By way of example, the motor may be a polyphase AC motor and the signals indicative of rotor position may be the BEMF voltages of the windings.
Preferably, the step of converting kinetic energy of the polyphase motor comprises boosting the BEMF voltage to produce the secondary DC source. To this end, the method may include: providing respective paths for current to flow between pairs of the windings of the polyphase motor such that the current ramps up during some periods of time (e.g., first periods of time); and interrupting the respective paths for current and providing other respective paths for the current to flow between the pairs of the windings of the polyphase motor such that the current ramps down during other periods of time (e.g., second periods of time). For example, the current may be circulated to the secondary DC source during at least one of the first and second periods of time. Preferably, the current bypasses the secondary DC source during the first periods of time.
By way of example, a pulse width modulation regulator circuit may be used to control the periods of time during which the respective paths are provided and interrupted in response to the voltage level of the secondary DC source. Alternatively, an aggregate ripple current of the current flowing through the respective paths may be used to control the periods of time during which the respective paths are provided and interrupted.
In accordance with one or more further aspects of the present invention, an apparatus includes: a voltage sensing circuit operable to monitor a level of a DC source that is used to provide an operating DC voltage to a control circuit and to provide DC bus voltage to a driver circuit for a polyphase motor, the control circuit being of a type that senses signals in windings of the polyphase motor to determine a rotor position thereof and to maintain synchronization therewith; a boost circuit operable to convert kinetic energy of the polyphase motor into a secondary DC source when the level of the DC source has fallen and reached a threshold; and a voltage regulator circuit operable to provide signaling to the boost circuit to regulate a voltage level of the secondary DC source such that it is operable to provide the operating DC voltage to the control circuit, and such that the control circuit is capable of maintaining synchronization with the polyphase motor while the DC source remains substantially at or below the threshold.
Preferably, the boost circuit is operable to boost the BEMF voltage on the windings of the polyphase motor to produce the secondary DC source. To this end, the boost circuit may include a plurality of commutation elements that are controlled to: provide respective paths for current to flow between pairs of the windings of the polyphase motor such that the current ramps up during some periods of time (e.g., first periods of time); and interrupt the respective paths for current and providing other respective paths for the current to flow between the pairs of the windings of the polyphase motor such that the current ramps down during other periods of time (e.g., second periods of time). Again, the current may be circulated to the secondary DC source during at least one of the first and second periods of time. Preferably, the current bypasses the secondary DC source during the first periods of time.
By way of example, the driver circuit may include respective pairs of high-side and low-side switches coupled in series across the DC bus and coupled at respective intermediate nodes to the windings of the polyphase motor, each switch including an anti-parallel diode thereacross. In such a case, the boost circuit is preferably operable to use at least some of the anti-parallel diodes to provide the paths for current to flow between the pairs of the windings of the polyphase motor.
Preferably, the commutation elements include respective commutating switches, coupled from the intermediate nodes to a common node of the low-side switches, to provide the paths for current to flow between the pairs of the windings of the polyphase motor, and for the current to ramp up during some periods of time. For example, the commutating switches may include respective diodes, each having an anode coupled to one of the intermediate nodes and having a cathode coupled to the common node of the low-side switches through a switch. Alternatively, the commutating switches may include respective transistors coupled from the intermediate nodes to the common node of the low-side switches. Still further, the commutating switches may be: (i) two or more of the low-side switches; (ii) two or more of the high-side switches; or (iii) one of the high-side switches and one of the low-side switches (in a manner where the DC bus voltage aids the BEMF voltage), which are operable to turn on to provide the paths for current to flow between the pairs of the windings of the polyphase motor, and for the current to ramp up during the first periods of time.
Preferably, the current is circulated to the secondary DC source at least one of the first and second periods of time. It is most preferred that the current bypasses the secondary DC source during the first periods of time.
Preferably, during a motoring mode, the control circuit is operable to provide commutation control signals to the driver circuit such that the windings are commutated with respect to the DC bus voltage to cause the polyphase motor to produce motoring torque; and at least one of the voltage sensing circuit and the voltage regulator circuit is operable to inhibit the control circuit from providing the motoring commutation control signals to the driver circuit while the DC source remains substantially at or below the threshold. Further, it is preferred that the at least one of the voltage sensing circuit and the voltage regulator circuit is operable to enable the control circuit to provide the motoring commutation control signals to the driver circuit when the DC source rises substantially to or above the threshold, wherein the enabling may be carried out without first stopping and restarting the polyphase motor.
Other advantages, features, and aspects of the invention will be apparent to one skilled in the art in view of the discussion herein taken in conjunction with accompanying drawings.